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The bars are lime green, perhaps chosen by a colour consultant for calming properties when the prison was built in 1969. Between them slips a mirror, an inmate's periscope, showing in reflection a haggard face.

Our guide for the day, Bryan Christie, has an easy manner with those he guards. After 30 years working his way up from correctional officer to Paremoremo manager, he says he misses his time on the floor.

"You can actually tell how the prison's doing just by listening. From inmate banter you can tell if there's trouble or if all's going well."

Today the inmate banter doesn't spell trouble, but neither does it spell any ease.

"You're treating us like animals in here, Bryan!" shouts the prison submariner.

"No, we're not," replies Christie.

The exchange - alleged abuse and reasoned denial - reaches specifics.

"We can buy Milo and noodles from the canteen," yells the prisoner, "but we're not allowed hot water."

"There's a reason for that," says Christie. During 1998 riots inmates improvised a hot-water hose to scald wardens. Prison management is well aware of the ingenious potential of some of their charges.

"Welcome to the human zoo!" comes the call from the cage.

It's the last thing I hear as Christie guides us away. "I just feel sorry for his victims," he mutters.

That prisoner is serving a long stretch, one of the longest in New Zealand legal history. He's not a pleasant man and neither are his surroundings. The length of his sentence and his crowded accommodation reflect a trend that is seeing our human zoo outgrowing its cage.

Minister of Corrections Paul Swain admits there is "great concern" at the size of the prison population. Corrections is able to house 6462 inmates, and at the end of March there were 6403 detainees.

This is no new crisis. At the start of October 1985, New Zealand's prison population exceeded capacity by 172 prisoners. The then assistant secretary for justice Mel Smith told the Herald, "When you are under that sort of accommodation pressure you start to have trouble."

The release of more than 1000 prisoners under the Criminal Justice Act brought temporary respite and the Justice Department announced it would close Wanganui City and Waikune prisons because they were no longer needed.

But the relief was short-lived. By the end of 1989, there were almost 3900 prisoners, up 1300 from 1985. Mel Smith, now Deputy Secretary of Justice, again told the Herald an increase in crime and jail sentences was putting pressure on already overcrowded prisons. Corrections was adding new units to Paremoremo and Rolleston, opening a prison in Hawkes Bay, buying land in Northland and looking for a site in South Auckland.

The explanation is simple: a slight rise in violent crime rates and longer sentences for those crimes. Labour and National governments have introduced legislation to ratchet up sentence length. Between 1985 and 1987, Labour made prison mandatory for many violent offences and extended non-parole periods.

The result: the prison population doubled between 1985 and 1999. Now it is projected by the Ministry of Justice to increase another third between 1999 and 2010, helped along by the Government's 2002 Bail, Parole and Sentencing Act.

Our imprisonment rate is already high by international standards We detain 155 people for every 100,000 inhabitants, ranking us seventh in the OECD. If present trends continue the rate will be up to 174 for every 100,000 by 2010, ranking us third, behind the United States and Poland.

MPs from across the spectrum know the public demands longer sentences. Progressive MP and former Corrections Minister Matt Robson says the Government is being "held hostage" on law and order and Act's Stephen Franks says politicians are being "forced by public opinion to be punitive and increase sentences".

Kathy Dunstall, secretary of the Canterbury branch of the Howard League for Penal Reform, says the system "is just about bursting at the seams" and points out the 2002 election saw "the complete convergence of all political parties" on the need for longer sentences. No one wanted to appear soft on crime following the 1999 referendum, and a victorious Labour Party was no exception.

Swain says: "I think that the public are genuinely alarmed and outraged, particularly at violent criminals, and they have demanded that the Government act on those and lock them up for longer."

Greg Newbold, senior lecturer in sociology at the University of Canterbury, says longer sentences are mainly about revenge.

The pressure is not just on politicians. Victoria University Professor of Criminology John Pratt also blames longer sentences on "a more punitive stance being taken by judges, often because that's what they believe the public is insisting on".

And the Government's Bail, Parole and Sentencing Acts, all passed in 2002, are adding to the squeeze. More people are being denied bail, so more people are being held in remand prisons. "Remand muster this year has increased past everyone's expectations," says acting general manager of Auckland Central Remand Prison Chris Burns.

The Bail Act came from "a lot of concern about people committing crimes on bail", says Swain but Public Prisons Service general manager Phil McCarthy says the act has caught more than just serious offenders. "Only about 50 per cent of people who are remanded in custody are ultimately given prison sentences."

Meanwhile, he says, "sentence lengths are increasing and the number of sentences of imprisonment are increasing".

Granting parole under new law is more at the discretion of the parole board and, says McCarthy, prisoners are now serving more of their sentence before being granted early release.

With prisons full already and demand set to steadily increase over the next six years, there are rocky times ahead.

"They send remand prisoners all around the country," she says. "I get calls from families, saying their mother has been flown away from them."

It's a story Dunstall has heard before: "Women with children up in Auckland are being fired down to Otago."

But Swain maintains the system is not in danger of collapse. "There is the ability to manage in the short term. For example in low-security areas we can do more double-bunking in cells."

Rising imprisonment levels have come at a formidable price. The Corrections Department budget increased 72 per cent between 1997 and 2004 from $361m to $620m. It is likely to hit close to $900m in 2006 with four new prisons being built in Northland, the Waikato, South Auckland and Otago. They will collectively house 1400 inmates.

Construction has been budgeted at $600m and the facilities are expected to cost $120m a year to operate.

"Taxpayers should have their mouths open at those kind of figures," says Swain. "The real issue is that most New Zealanders want criminals locked up, and locked up for a long time, but there's a huge cost that comes with that."

It's a cost that Garth McVicar of the Sensible Sentencing Trust is prepared to pay.

"Obviously we were aware there were going to be some horrific costs involved," he says. "We took the view that there's horrific costs to victims on the other side of the ledger. I see no alternatives but to make some severe sacrifices to reduce the level of violent crime."

Franks cites American studies showing recidivist offenders cost $150,000 a year to the community, and concludes "it's still going to be worth locking them up".

But Swain says the Government is caught in a bind. "The public are demanding two things: one, to lock people up, and two, to make sure that people fit back normally into society. The second part is hard in light of the first."

One factor law and order campaigners cite in favour of more prisons and longer sentences is offenders' tendency to reoffend. Corrections figures indicate 36 per cent of inmates who are released will again be behind bars within two years.

And while the equivalent figure for the British correctional system is 57 per cent, this difference "doesn't mean we're doing well", says Pratt. "On the one hand you want to send people there to punish them, but going to prison is a debilitating experience in so many ways you're also ensuring that they come back."

A strong body of research and opinion argues that locking prisoners up for longer increases the chances they'll re-offend. It's a view that those involved in running prisons tend to support.

"I'm not convinced whether that's [longer sentences] actually going to have a long-term effect on reoffending," he says. "I think it's quite a populist sort of cry to have longer sentences. As a member of the public I can understand it, because I'm also a member of the public."

Which brings us to the other chief purpose of prisons: rehabilitation.

Despite more than a third of prisoners returning to his care within two years, McCarthy says that "certain programmes, given to certain inmates, work. If you get your programmes right, you can reduce your re-imprisonment rates by about 10 percentage points."

One example of successful rehabilitation, says Swain, is the Te Marama sex offenders unit in Christchurch, which has been "getting a recidivism rate of around 8 per cent, which by international standards is quite outstanding".

Rehabilitation within prison, as Swain and McCarthy note, will never reduce recidivism to zero but even marginal success, a percentage point or two, will have a dramatic effect on our burgeoning prison and crime statistics.

The new prisons are part of a wider strategy to site prisons closer to where prisoners' families live, with better prospects of community support and, it is hoped, lower recidivism.

But the fact remains that the instant the new prisons open their doors, they'll be full. Their capacity caters only for projected increases.

In 2010, the prison system will be at 94 per cent capacity. If the growth in imprisonment this decade is repeated, by 2014 we will be facing another round of prison construction.

"In the long run the answer to this is not just simply building new prisons," Swain says. And the answers to long-term questions of law and order are not simple.

"We haven't yet got the debate on to what works, as opposed to what's popular," says Robson. "An enormous fault of not being able to move forward on this lies with us - the politicians."

Fear, contend both Pratt and Dunstall, is perpetuated by saturation media coverage of high-profile crimes.

Late last year the Ministry of Justice released a survey analysing perceptions of crime. It concluded that most respondents overestimated levels of crime and underestimated the length of time a criminal would spend in prison. It is, perhaps, no wonder the public feels more needs to be done.

Of course, there are some who feel we are already on the right track. McVicar expects the prison population to peak and start declining within four years.

On the other hand Franks thinks the situation will further deteriorate, and the prison population will continue to climb.

"We're going to get the worst of both worlds. The Government hasn't been willing to send an unequivocal message, as other countries have, to really cut crime." His answer? Even longer sentences, and the abolition of parole.

But some of Newbold's earlier research found that harsher sentences had no apparent effect on offending. Between 1985 and 1997, as sentence lengths almost doubled, "the number of persons convicted of violent offences more than doubled, with the greatest growth occurring in serious violence", he says.

As for Swain: "I hope I'm not just remembered as the one that built the prisons." He hopes he's remembered for successfully implementing rehabilitation and reintegration programmes.

"We're aiming for a point where we wouldn't need to build any new prisons. Then the work that we're doing with other agencies, across the board, will have worked."